Energy efficient roof systems increasingly are in demand because of rising energy costs, evolving building codes and greater sensitivity to the effects of urban heat islands. Energy-efficient roofing materials result in cooler roof surfaces and less energy spent for air conditioning. From an environmental standpoint, reduced cooling costs translate to reduced fuel usage, less power-plant emissions and fewer particulate matter in the air. Energy efficient roof coatings reduce roof insulation thickness requirements and ceiling plenum construction.
In addition, some energy codes have begun to include minimum requirements for reflectivity and emissivity (i.e., a surface's ability to emit heat). For example, the Cool Roof Rating Council has developed a system to evaluate and label roof coverings using independent testing labs so energy performance values for all roof coverings can be included in energy codes. As a result of this demand and media attention about ENERGY STAR® rating and reflectivity, acrylic coatings have been used as finish coats for modified bitumen roof systems and maintenance coatings for existing roof systems.
White, water-based acrylic coatings have been found to provide the highest reflectivity and longevity. White reflective coatings also typically minimize heat damage to roof membranes, increasing their expected service lives. Acrylic coatings primarily are formulated with pigments, acrylic polymers and water. There may be other additives, such as fibers for reinforcement, glycol for freeze thaw resistance, intumescant or other fire-retardant additives, or biocides to prevent fungal growth in the container. ENERGY STAR® listings are specific to a coating formula. That is, coating formulation changes must be tested and recertified before establishing the ENERGY STAR® listing for that coating.
With prior art white, water-based acrylic coatings problems have occurred in maintaining roof surface reflectivity. Reflectivity decreases the most during the first year of a roof's life. After three years, the rate that reflectivity declines is typically less significant. Changes in reflectivity are primarily related to changes with the coating itself (e.g., coating-erosion or cracking) and/or minimally related to accumulation of particulate matter (e.g., dirt) from atmospheric fallout. Depending on the geographic exposure and how well roof surfaces drain, keeping roof surfaces white and preventing premature failure from cracking and peeling can be a significant challenge and result in major maintenance expenditures for owners. Maintaining reflectivity may involve regular cleaning, regular restoration of reflective coatings, and regular application of biocides and/or fungicides. There remains a need for improved coatings with greater reflectivity, energy efficacy and durability.
Prior art acrylic coatings are applied directly to granule-surfaced modified bitumen roof membranes on new roof systems or as restorative coatings. However, granules are difficult to coat because of their rough, uneven surface areas. Moisture and air pockets can be trapped under the acrylic coating and lead to blisters or pinholes in the cured acrylic coating. Consequently, application of a compatible primer to the granule surface before coating application is required. Inconsistent coverage and potential cracking of areas where the coating is applied too heavily are additional problems related to application of previous acrylic coatings.
Prior art coatings require application to the roofing membrane subsequent to placement of the modified bitumen membranes. Application requires special equipment such as a pressure washer, paddle mixer and spray rig as well as personal protective equipment. Pressure washing removes embedded dirt, chalking, carbon black and poorly adhered material. A paddle mixer is required as the coating must be completely stirred to ensure proper polymer dispersion because the solids may have settled at a container's bottom. Hence, there is a need for coating compositions that can be easily and effectively applied without the need for special equipment.
Acrylic coatings develop strength and adhesion as they cure during installation. When an acrylic coating is applied, two physical changes must occur: water must evaporate from the applied coating film for initial drying and acrylic polymers must fuse together for final cure. Consequently, for application purposes, multiple thin coats promote water evaporation, polymer dispersion, and help eliminate pinholes, voids or thin spots.
Application of water-based acrylic coatings is influenced by changing weather conditions. Virtually all parts of North America have some application limitations as a result of cold weather, daily rainstorms, high humidity and/or fog, or reduced daylight hours during winter. Rain on an uncured coating will cause a partial or total coating run-off. Problems occur when an acrylic coating is specified on a construction project without regard to the time of year the coating is to be installed.
Therefore, two or more successive coats of the coating are often necessary. Further, the drying of the coating is influenced by weather conditions. Cold temperatures and lack of sunlight decrease the freshly applied coating's evaporation. Water in the coating film closest to the membrane diffuses through slowly. Coatings exposed to water conditions during the drying or coating period may soften, lift and debond from the surface. This often requires cleaning of the surface and reapplication of the coating. The final cure takes place during the first few weeks after application and is essential to the coating's long term performance. Wet weather and cooler temperatures inhibit final cure and may inhibit proper fusing. Consequently, acrylic coating applications cannot be attempted on roofing projects from late fall to early spring in most North American areas.
Hence, there is a need for new and improved coating compositions that may be applied in-plant during manufacture of the roll roofing membrane. In particular, a coating composition is needed that is reflective, energy efficient (meeting today's Energy Star® criteria) as well as durable and easy to apply, and which is not vulnerable to the effects of moisture and cold temperatures during the curing process.